Portable electronic device being capable of contactless charge

ABSTRACT

A portable electronic device that can operate even when electric power supplied through contactless charge by electromagnetic induction is low is provided. The portable electronic device includes a reflective liquid crystal display which includes a transistor including an oxide semiconductor, a power source portion which includes a rechargeable battery capable of charge by contactless charge, and a signal processing portion which includes a nonvolatile semiconductor memory device. In the portable electronic device, electric power stored in the rechargeable battery is used in the reflective liquid crystal display and the signal processing portion.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The technical field of the present invention relates to a portableelectronic device capable of contactless charge.

2. Description of the Related Art

Portable electronic devices such as e-book readers that enable readingby displaying computerized book data on a display have been spread. Suchdevices are assumed to be used portably, and can be driven by batteriesincorporated in main bodies. Therefore, these devices are preferablysupplied with electric power by using an external power supply device.

In Patent Document 1, an e-book reader supplied with power supplyvoltage by using an AC adapter is suggested.

In Patent Document 2, a display device is suggested. The display deviceincludes a display portion, a console portion for operation and inputfrom external, an antenna portion for transmitting and receiving awireless signal, a controller portion for controlling a signal that isinput in the console portion and a signal that is transmitted andreceived by the antenna portion, and a battery portion for converting awireless signal received by the antenna portion into electric power andfor storing it as electric power for driving the display portion.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2007-147871-   [Patent Document 2] Japanese Published Patent Application No.    2008-181108

SUMMARY OF THE INVENTION

In order to supply power supply voltage from an AC adapter, a contactsuch as a metal electrode needs to be provided on a portable electronicdevice side. The contact needs durability. In addition, it is necessaryto take measures against contact failure or generation of leakage due toshort circuit, moisture, or the like.

In addition, in order that a portable electronic device can operate evenwhen electric power supplied through contactless charge byelectromagnetic induction is low, power consumption of the portableelectronic device needs to be reduced.

An object of one embodiment of the present invention is to provide aportable electronic device that can operate even when electric powersupplied through contactless charge by electromagnetic induction is low.

One embodiment of the present invention is a portable electronic deviceincluding a reflective liquid crystal display, a power source portion,and a signal processing portion. The reflective liquid crystal displayincludes a transistor including an oxide semiconductor, the power sourceportion includes a rechargeable battery capable of charge by contactlesscharge, and the signal processing portion includes a nonvolatilesemiconductor memory device.

Another embodiment of the present invention is a portable electronicdevice including a reflective liquid crystal display, a power sourceportion, and a nonvolatile semiconductor memory device. The reflectiveliquid crystal display includes a first transistor including a firstoxide semiconductor, the power source portion includes a rechargeablebattery capable of charge by contactless charge, and the nonvolatilesemiconductor memory device includes a second transistor, a thirdtransistor, and a capacitor. A first terminal of the second transistoris electrically connected to a first wiring, a second terminal of thesecond transistor is electrically connected to a second wiring, and agate of the second transistor is electrically connected to a firstterminal of the third transistor and one electrode of the capacitor. Asecond terminal of the third transistor is electrically connected to athird wiring, and a gate of the third transistor is electricallyconnected to a fourth wiring. The other electrode of the capacitor iselectrically connected to a fifth wiring.

The third transistor may be a transistor including a second oxidesemiconductor. The second oxide semiconductor may be the same as thefirst oxide semiconductor or different from the first oxidesemiconductor.

Electric power stored in the rechargeable battery may be used in thereflective liquid crystal display and the signal processing portion.

The power source portion may include a solar cell.

The oxide semiconductor is intrinsic or substantially intrinsic, and theoff-state current per unit channel width of a transistor is lower thanor equal to 100 aA/μm (“a” is 10⁻¹⁸), preferably lower than or equal to1 aA/μm, more preferably lower than or equal to 1 zA/μm (“z” is 10⁻²¹)at room temperature. Note that in this specification, “intrinsic” meansthe state of a semiconductor with a carrier concentration of lower than1×10¹²/cm³, and “substantially intrinsic” means the state of asemiconductor with a carrier concentration of higher than or equal to1×10¹²/cm³ and lower than 1×10¹⁴/cm³.

Since contactless charge can be performed, a contact such as a metalterminal on a portable electronic device side does not need to beprovided. In addition, a portable electronic device that can operateeven when electric power supplied through contactless charge byelectromagnetic induction is low can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a portable electronic device.

FIG. 2 is a drawing of a structure example of the portable electronicdevice and a battery charger.

FIG. 3 is a diagram of an example of a circuit structure of anonvolatile semiconductor memory device.

FIGS. 4A and 4B are schematic views of an example of a display panel ofa liquid crystal display device, and FIG. 4C is a chart of an example ofa method for driving the display panel.

FIG. 5 is a block diagram of a portable electronic device.

FIG. 6 is a diagram of a connection relation between an antenna, a solarcell, and a rechargeable battery.

FIG. 7 is a drawing of a structure example of the portable electronicdevice.

FIGS. 8A and 8B are drawings of a structure example of a transistor.

FIGS. 9A to 9E are drawings showing an example of a method formanufacturing the transistor.

FIG. 10 is a graph showing electric characteristics of transistors.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention disclosed herein will be described belowwith reference to the drawings. Note that the invention is not limitedto the following description, and those skilled in the art can easilyunderstand that modes and details of the invention can be changed invarious ways without departing from the purpose and the scope of theinvention. Accordingly, the invention should not be interpreted as beinglimited to the following description of the embodiments.

Embodiment 1

In this embodiment, a structure of hardware of a portable electronicdevice of this embodiment is described with reference to FIG. 1, FIG. 2,FIG. 3, and FIGS. 4A to 4C.

FIG. 1 is the block diagram of the portable electronic device of thisembodiment. A portable electronic device 1 includes a display 10, apower source portion 20, and a signal processing portion 30. Note thatan example in which the portable electronic device 1 is used as ane-book reader (also referred to as an e-book terminal device) isdescribed below.

(Power Source Portion 20)

The power source portion 20 includes an antenna 21, a rectifier circuit22, a rechargeable battery 23, and a DC-DC converter 24.

(Antenna 21)

The antenna 21 is a power-receiving coil for contactless charge.

In FIG. 2, a structure example of the portable electronic device and abattery charger are illustrated. As illustrated in FIG. 2, the portableelectronic device 1 includes the antenna 21, and a battery charger 40includes a power-sending coil 41. The contactless charge is performed bycontactless transmission of electric power by electromagnetic inductionbetween the coils, and a metal terminal is not provided between thecoils.

The contactless charge does not need a contact such as a metal terminalfor charging between the portable electronic device 1 and the batterycharger 40. Therefore, there is no contact failure. Alternatively,leakage or the like due to short circuit, moisture, or the like hardlyoccurs.

(Rectifier Circuit 22)

Electric power received by the antenna 21 is rectified in the rectifiercircuit 22 and is stored in the rechargeable battery 23.

(Rechargeable Battery 23)

The rechargeable battery 23 is a capacitor that can store electricity bycharging and can be used repeatedly as a battery. As the rechargeablebattery 23, for example, a lithium ion battery, a lithium ion capacitor,or the like can be used. In addition, the rechargeable battery 23includes a control circuit for preventing overcharge and overdischarge.

(DC-DC Converter 24)

Electric power stored in the rechargeable battery 23 is transmitted tothe display 10 and the signal processing portion 30 via the DC-DCconverter 24 and is used as power supply voltage therein.

(Signal Processing Portion 30)

The signal processing portion 30 includes an antenna 31, a downconverter 32, a signal processing circuit 33, an NVM (a nonvolatilememory) 34, and a display controller 35.

(Antenna 31)

The antenna 31 transmits a request for starting download of computerizedbook data (referred to as “e-book data” below) which is selected by auser of the portable electronic device 1 to a server, and receives thee-book data sent from the server in response to the request.

(Down Converter 32 and Signal Processing Circuit 33)

The e-book data received by the antenna 31 is down-converted in the downconverter 32 and converted into a baseband signal. The baseband signalis processed in the signal processing circuit 33.

(NVM 34)

Since the amount of the e-book data received by the antenna 31 is largerthan that of data for one screen of the display, the received data isstored in the NVM 34.

Here, an example of the structure of a circuit of a nonvolatilesemiconductor memory device that can be applied to the NVM 34 isdescribed with reference to FIG. 3.

A nonvolatile semiconductor memory device illustrated in FIG. 3 includesa transistor 71, a transistor 72, and a capacitor 73.

A transistor including a material other than an oxide semiconductor issuitable for the transistor 71. Examples of the material other than theoxide semiconductor include single crystal silicon, crystalline silicon,and the like. The transistor including the material other than the oxidesemiconductor can operate at high speed, so that data can be read fromthe nonvolatile semiconductor memory device at high speed.

In contrast, a transistor including an oxide semiconductor is suitablefor the transistor 72. A transistor including an oxide semiconductor hasa characteristic of extremely low off-state current. Thus, data storedin the nonvolatile semiconductor memory device can be retained for along time. Therefore, refresh operation is unnecessary or performed lessoften, and power consumption of the nonvolatile semiconductor memorydevice can be reduced.

A gate of the transistor 71 is connected to one of a source and a drainof the transistor 72, a source of the transistor 71 is connected to awiring 50 (a source line), and a drain of the transistor 71 is connectedto a wiring 51 (a bit line). The other of the source and the drain ofthe transistor 72 is connected to a wiring 52 (a first signal line), anda gate of the transistor 72 is connected to a wiring 53 (a second signalline). In addition, one electrode of the capacitor 73 is connected tothe gate of the transistor 71 and the one of the source and the drain ofthe transistor 72, and the other electrode of the capacitor 73 isconnected to a wiring 54 (a word line).

Next, data write operation to the nonvolatile semiconductor memorydevice illustrated in FIG. 3 is described.

First, voltage is applied to the gate of the transistor 72, which isconnected to the wiring 53, so that the transistor 72 is turned on. As aresult, the wiring 52 and the transistor 71 are brought into conduction,and voltage is applied to the gate of the transistor 71 and thecapacitor 73, so that data is written.

After that, the transistor 72 is turned off, so that the voltage appliedto the gate of the transistor 71 is held and the data is stored. At thistime, when a voltage at which the transistor 71 is turned on is appliedto the gate of the transistor 71, the on state of the transistor 71 isheld for a long time. In contrast, when a voltage at which thetransistor 71 is turned off is applied to the gate of the transistor 71,the off state of the transistor 71 is held for a long time.

Next, data read operation from the nonvolatile semiconductor memorydevice illustrated in FIG. 3 is described.

In the case of the on or off state of the transistor 71, a constantvoltage is applied to the wiring 50 and a read voltage is applied to thewiring 54. As a result, in the case where the transistor 71 is on, thevoltage of the wiring 51 varies. In contrast, in the case where thetransistor 71 is off, the voltage of the wiring 51 does not vary.Therefore, by comparing the voltages between the wiring 50 and thewiring 51, the data stored in the nonvolatile semiconductor memorydevice can be read.

Data rewrite operation to the nonvolatile semiconductor memory deviceillustrated in FIG. 3 is similar to the data write operation.

(Display Controller 35)

The display controller 35 transmits data whose content is to bedisplayed on the display 10.

Most of the e-book data is still-image data; thus, the data rewritespeed does not need to be very high. Accordingly, high-speed operationof the down converter 32, the signal processing circuit 33, the NVM 34,and the display controller 35 is not required. Therefore, these circuitscan be driven with a low voltage. Thus, even when electric powersupplied through contactless charge by electromagnetic induction is low,the portable electric device 1 can operate.

(Display 10)

The display 10 displays content of the e-book data. Note that with theuse of a display that does not emit light by itself, such as areflective liquid crystal display or an electrophoretic display, powerconsumption of the display 10 can be set to 10 mW or less. Thus, evenwhen electric power supplied through contactless charge byelectromagnetic induction is low, the portable electric device 1 canoperate.

Here, an example of a circuit structure that can be applied to a displayis described with reference to FIGS. 4A and 4B.

FIG. 4A is a schematic view of an example of a display panel of a liquidcrystal display device. A display panel 60 includes a pixel portion 61,a gate signal line 62, a gate signal line driver circuit 62D, a datasignal line 63, a data signal line driver circuit 63D, a pixel 64, acommon electrode 65, a capacitor line 66, and a terminal portion 67.

FIG. 4B is a view of the pixel 64 illustrated in FIG. 4A. The pixel 64includes a transistor 75 including an oxide semiconductor, a liquidcrystal element 76, and a storage capacitor 77.

Next, an example of a method for driving the display panel is describedwith reference to FIG. 4C.

First, in order to write an image signal BK/W to the pixel, thetransistor 75 is turned on, and a period T (hereinafter referred to as a“writing period T1”) during which a voltage based on the image signal isapplied to a pixel electrode of the liquid crystal element 76 isprovided. In the writing period T1, a driver circuit control signal issupplied to a driver circuit of the display 10 and the displaycontroller 35, so that these circuits operate.

After the writing period T1, voltage V_(pix) is generated in the pixelelectrode of the liquid crystal element 76. Then, the transistor 75 isturned off, so that the voltage V_(pix) is held in the pixel electrodeof the liquid crystal element 76.

In a subsequent period T2 (hereinafter referred to as a “holding periodT2”) during which the voltage V_(pix) is held in the pixel electrode ofthe liquid crystal element 76, the image signal BK/W is not written. Inaddition, the driver circuit control signal is not supplied to thedriver circuit of the display 10 and the display controller 35, so thatthese circuits do not operate.

The length of the holding period T2 varies depending on an off-statecurrent I₇₅ of the transistor 75 and a current I₇₆ flowing through theliquid crystal element 76. In order to prevent a screen flicker due tovariation in these currents, refresh operation by which data displayedon the screen is rewritten at regular intervals is necessary.

Note that the off-state current I₇₅ of the transistor 75 including anoxide semiconductor is extremely low. Therefore, the length of theholding period T2 is dependent only on the current I₇₆ flowing in theliquid crystal element 76. Thus, the number of rewrite operation of datato be displayed on the screen can be approximately 1/1000 the number ofgeneral rewrite operation (60 times per second).

As described above, in the holding period T2, the operation of thedriver circuit of the display 10 and the display controller 35 can bestopped. Thus, power consumption of the display 10 and the displaycontroller 35 can be approximately 1/1000 general power consumption.

Next, the oxide semiconductor included in the transistor is described.

Impurities such as hydrogen, moisture, hydroxyl group, or hydroxide(also referred to as a hydrogen compound) which serve as donors areintentionally removed from the oxide semiconductor included in thetransistor, and then oxygen which is simultaneously reduced in the stepof removing these impurities is supplied, so that the oxidesemiconductor is highly purified and becomes electrically i-type(intrinsic). This is for suppressing variation in electriccharacteristics of the transistor.

Hydrogen contained in the oxide semiconductor is removed as much aspossible; thus, the carrier density of the oxide semiconductor is lessthan 1×10¹⁴/cm³, preferably less than 1×10¹²/cm³, or more preferablyless than 1×10¹⁰/cm³.

In an oxide semiconductor, which is a wide band gap semiconductor, thedensity of the minority carrier is low and the minority carrier isdifficult to be induced. Thus, in the transistor including the oxidesemiconductor, tunnel current is difficult to be generated;consequently, off-state current is difficult to flow.

In addition, impact ionization and avalanche breakdown are less likelyto occur in the transistor including the oxide semiconductor, which is awide band gap semiconductor. Therefore, the transistor including theoxide semiconductor has resistance to hot carrier deterioration. The hotcarrier deterioration is mainly caused by an increase in the number ofcarriers due to avalanche breakdown and by injection of the carriersaccelerated to high speed to the gate insulating film.

Note that the “off-state current” in this specification means currentflowing between a source and a drain of an n-channel transistor withpositive threshold voltage (Vth) when a given gate voltage of higherthan or equal to −20 V and lower than or equal to −5 V is applied atroom temperature. Note also that “room temperature” is higher than orequal to 15° C. and lower than or equal to 25° C.

The transistor including the oxide semiconductor disclosed in thisspecification has a current value per micrometer channel width (1 μm) of100 aA/μm or less, preferably 1 aA/μm or less, more preferably 10 zA/μmor less at room temperature.

As described above, by using an oxide semiconductor that is highlypurified and becomes electrically i-type (intrinsic), a transistor withan extremely small off-state current value can be provided. A testelement group (also referred to as a TEG) is manufactured and themeasurement results of off-state current characteristics are describedbelow.

In the TEG, two hundred transistors each with L/W=3 μm/50 μm (thethickness d was 30 nm) are connected in parallel to provide a transistorwith L/W=3 μm/10000 μm. Note that W represents the channel width and Lrepresents the channel length.

FIG. 10 is a graph showing electric characteristics (log(I_(d))−V_(g))of the transistors provided in the TEG In FIG. 10, the horizontal axisrepresents gate voltage V_(g) [V], and the vertical axis representsdrain current I_(d) [A]. Note that the substrate temperature was roomtemperature, and the voltage V_(d) between a source and a drain waseither 1 V (a dotted line in the graph) or 10 V (a solid line in thegraph). In the above condition, the voltage V_(g) between the source andthe gate was varied from −20 V to +20 V, and transfer characteristics ofthe drain current I_(d) were measured.

As shown in FIG. 10, the off-state current value of the transistor witha channel width W of 10000 μm was 1×10⁻¹³ A or less when V_(d) was 1 Vor 10 V. This value is less than or equal to the resolution (100 fA) ofa measurement device (a semiconductor parameter analyzer, Agilent 4156Cmanufactured by Agilent Technologies, Inc.). This off-state currentvalue corresponds to 10 aA/μm per micrometer channel width (1 μm).

Embodiment 2

In this embodiment, a structure of hardware of a portable electronicdevice which is different from the structure of hardware of the portableelectronic device of Embodiment 1 is described with reference to FIG. 5,FIG. 6, and FIG. 7.

FIG. 5 is a block diagram of the portable electronic device of thisembodiment. A portable electronic device 2 has a structure in which asolar cell 25 is added to the power source portion 20 of the portableelectronic device 1 illustrated in FIG. 1. With the structure includingthe solar cell 25, when the portable electronic device 2 is exposed tosunlight or illumination light, electricity can be stored in therechargeable battery 23 with the use of the solar cell 25.

In FIG. 6, a connection relation between the antenna, the solar cell,and the rechargeable battery are illustrated. Backflow prevention diodesare provided between the antenna 21 and the rechargeable battery 23 andbetween the solar cell 25 and the rechargeable battery 23.

A structure example of the portable electronic device including thesolar cell is described with reference to FIG. 7.

The portable electronic device 2 illustrated in FIG. 7 has a structurein which e-book data is displayed on the display 10 in the state wherethe portable electronic device 2 is open. Thus, when the solar cell 25is provided so as to be exposed outside in the state where the portableelectronic device 2 is open, the solar cell 25 can perform charge whilethe e-book data is displayed on the display 10.

A display that does not emit light by itself, such as a reflectiveliquid crystal display or an electrophoretic display, is particularlysuitable for the display 10 because the solar cell 25 can performcharge.

Embodiment 3

In this embodiment, a structure example of a transistor including anintrinsic or substantially intrinsic oxide semiconductor and an exampleof a manufacturing method thereof are described with reference to FIGS.8A and 8B and FIGS. 9A to 9E.

FIGS. 8A and 8B are an example of a plan structure and an example of across-sectional structure of a transistor. FIG. 8A is a plan view of atop gate transistor. FIG. 8B is a cross-sectional view of a portiontaken along straight line C1-C2 in FIG. 8A.

A transistor 410 includes, over a substrate 400, an insulating layer407, an oxide semiconductor layer 412, a first electrode 415 a, a secondelectrode 415 b, a gate insulating layer 402, and a gate electrode 411.The first electrode 415 a is in contact with a wiring layer 414 a, andthe second electrode 415 b is in contact with a wiring layer 414 b.

Note that the transistor 410 illustrated in FIGS. 8A and 8B is a singlegate transistor; however, the structure of the transistor is not limitedthereto. For example, a multi-gate transistor may be applied.

Next, a process of manufacturing the transistor 410 is described withreference to FIGS. 9A to 9E.

First, the insulating layer 407 serving as a base film is formed overthe substrate 400. The insulating layer 407 is preferably formed whilemoisture remaining in a treatment chamber is removed. This is forpreventing hydrogen, water, hydroxyl group, hydroxide, or the like frombeing contained in the insulating layer 407.

Then, an oxide semiconductor layer is formed over the insulating layer407 by a sputtering method. Note that before the oxide semiconductorlayer is formed, the substrate 400 over which the insulating layer 407is formed is preferably preheated. This is for preventing hydrogen,moisture, and hydroxyl group from being contained in the oxidesemiconductor layer as much as possible. By the preheating, impuritiessuch as hydrogen or moisture adsorbed on the substrate 400 areeliminated and exhausted.

As a target for forming the oxide semiconductor layer, a metal oxidetarget containing zinc oxide as its main component can be used. Forexample, a target with a composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:1,that is, In:Ga:Zn=1:1:0.5 can be used. Alternatively, a target having acomposition ratio of In:Ga:Zn=1:1:1 or In:Ga:Zn=1:1:2 can be used.

Alternatively, a target of a metal oxide such as an In—Sn—Ga—Zn—O-basedmetal oxide, an In—Sn—Zn—O-based metal oxide, an In—Al—Zn—O-based metaloxide, an Sn—Ga—Zn—O-based metal oxide, an Al—Ga—Zn—O-based metal oxide,an Sn—Al—Zn—O-based metal oxide, an In—Zn—O-based metal oxide, anSn—Zn—O-based metal oxide, an Al—Zn—O-based metal oxide, a Zn—Mg—O-basedmetal oxide, an Sn—Mg—O-based metal oxide, an In—Mg—O-based metal oxide,an In—O-based metal oxide, an Sn—O-based metal oxide, or a Zn—O-basedmetal oxide can be used.

Note that as the oxide semiconductor layer, a thin film containing anoxide semiconductor expressed by InMO₃(ZnO)_(m) (m>0) can be used. Here,M represents one or more metal elements selected from Ga, Al, Mn, andCo. For example, Ga, Ga and Al, Ga and Mn, or Ga and Co can be given asM.

The oxide semiconductor layer is processed in a first photolithographystep, so that an island-shaped oxide semiconductor layer 412 is formed(see FIG. 9A). After that, in order to remove hydrogen, water, andhydroxyl group from the oxide semiconductor layer 412, the substrate 400over which the oxide semiconductor layer 412 is formed is put into anelectric furnace and subjected to heat treatment. This heat treatmenthas an effect of dehydration or dehydrogenation on the oxidesemiconductor layer 412.

The temperature of the heat treatment is higher than or equal to 400° C.and lower than or equal to 750° C., preferably higher than or equal to400° C. and less than the strain point of the substrate. In addition,the atmosphere of the heat treatment does not contain water, hydrogen,or the like.

After the heat treatment, successive heat treatment is preferablyperformed in an oxygen atmosphere or an atmosphere containing nitrogenand oxygen (e.g., nitrogen to oxygen is 4 to 1 in volume ratio). This isfor repairing oxygen deficiency generated in the oxide semiconductorlayer 412.

In FIG. 9B, a state where the first electrode 415 a and the secondelectrode 415 b are formed over the insulating layer 407 and the oxidesemiconductor layer 412 is illustrated. The first electrode 415 afunctions as one of a source electrode and a drain electrode. The secondelectrode 415 b functions as the other of the source electrode and thedrain electrode.

In FIG. 9C, a state where the gate insulating layer 402 is formed overthe insulating layer 407, the oxide semiconductor layer 412, the firstelectrode 415 a, and the second electrode 415 b is illustrated. Notethat it is preferable that hydrogen be not contained in an atmosphereused for forming the gate insulating layer 402.

In FIG. 9D, a state where an opening 421 a reaching the first electrode415 a and an opening 421 b reaching the second electrode 415 b areformed by removal of part of the gate insulating layer 402 isillustrated.

In FIG. 9E, a state where the gate electrode 411, the first wiring layer414 a, and the second wiring layer 414 b are formed over the gateinsulating layer 402 and in the opening 421 a and the opening 421 b isillustrated.

In the above manner, a transistor including an intrinsic orsubstantially intrinsic oxide semiconductor can be manufactured.

This application is based on Japanese Patent Application serial no.2010-010382 filed with Japan Patent Office on Jan. 20, 2010, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. An electronic device comprising: a displayportion having a pixel including a first transistor; a rechargeablebattery being capable of charge by contactless charge; and a memorydevice being configured to store data to be applied to the displayportion, wherein the rechargeable battery is configured to supplyelectric power to the display portion and the memory device, and whereinthe memory device includes: a second transistor, one of a source and adrain of the second transistor being electrically connected to a bitline; a third transistor, one of a source and a drain of the thirdtransistor being electrically connected to a gate of the secondtransistor; and a capacitor, a first electrode of the capacitor beingelectrically connected to the gate of the second transistor and a secondelectrode of the capacitor being electrically connected to a word line.2. The electronic device according to claim 1, wherein the firsttransistor has an off-state current of lower than or equal to 10 aA/μmwhen a voltage between a source and a drain of the first transistor is1V.
 3. The electronic device according to claim 1, wherein the firsttransistor has an off-state current of lower than or equal to 10 aA/μmwhen a voltage between a source and a drain of the first transistor is10 V.
 4. The electronic device according to claim 1, wherein the displayportion comprises a liquid crystal display device.
 5. The electronicdevice according to claim 1, further comprising a first antennaelectrically connected to the rechargeable battery, wherein the firstantenna is capable of receiving electric power.
 6. The electronic deviceaccording to claim 1, further comprising a signal processing portioncomprising the memory device.
 7. The electronic device according toclaim 6, wherein the signal processing portion further comprises asecond antenna, and wherein the second antenna is capable of receivingthe data.
 8. The electronic device according to claim 1, furthercomprising a driver circuit and a display controller electricallyconnected to the display portion, wherein operation of the drivercircuit and the display controller is stopped during a holding periodafter writing an image signal to the pixel.
 9. The electronic deviceaccording to claim 1, wherein the other of the source and the drain ofthe second transistor is electrically connected to a first wiring,wherein the other of the source and the drain of the third transistor iselectrically connected to a second wiring, and wherein a gate of thethird transistor is electrically connected to a third wiring.
 10. Theelectronic device according to claim 1, wherein a channel forming regionof each of the first transistor and the third transistor comprises anoxide semiconductor.
 11. An electronic device comprising: a displayportion having a pixel including a first transistor; a power sourceportion including a first antenna being capable of receiving electricpower by contactless charge and a rechargeable battery beingelectrically connected to the first antenna; and a signal processingportion including a second antenna being capable of receiving data, asignal processing circuit being electrically connected to the secondantenna, and a memory device being electrically connected to the signalprocessing circuit, wherein the rechargeable battery is configured tosupply electric power to the display portion and the signal processingportion, and wherein the memory device includes: a second transistor,one of a source and a drain of the second transistor being electricallyconnected to a bit line; a third transistor, one of a source and a drainof the third transistor being electrically connected to a gate of thesecond transistor; and a capacitor, a first electrode of the capacitorbeing electrically connected to the gate of the second transistor and asecond electrode of the capacitor being electrically connected to a wordline.
 12. The electronic device according to claim 11, wherein the firsttransistor has an off-state current of lower than or equal to 10 aA/μmwhen a voltage applied between a source and a drain of the firsttransistor is 1V.
 13. The electronic device according to claim 11,wherein the first transistor has an off-state current of lower than orequal to 10 aA/μm when a voltage applied between a source and a drain ofthe first transistor is 10 V.
 14. The electronic device according toclaim 11, wherein the third transistor has an off-state current of lowerthan or equal to 10 aA/μm when a voltage between the source and thedrain is 1V.
 15. The electronic device according to claim 11, whereinthe third transistor has an off-state current of lower than or equal to10 aA/μm when a voltage between the source and the drain is 10 V. 16.The electronic device according to claim 11, wherein the display portioncomprises a liquid crystal display device.
 17. The electronic deviceaccording to claim 11, wherein the power source portion further includesa rectifier circuit being electrically connected to the first antenna.18. The electronic device according to claim 11, wherein a channelformation region of the first transistor comprises an oxidesemiconductor.
 19. The electronic device according to claim 11, whereina channel formation region of the second transistor comprises silicon.20. The electronic device according to claim 11, further comprising adriver circuit and a display controller electrically connected to thedisplay portion, wherein operation of the driver circuit and the displaycontroller is stopped during a holding period after writing an imagesignal to the pixel.
 21. The electronic device according to claim 11,wherein the other of the source and the drain of the second transistoris electrically connected to a second first wiring, wherein the other ofthe source and the drain of the third transistor is electricallyconnected to a second wiring, and wherein a gate of the third transistoris electrically connected to a third wiring.
 22. The electronic deviceaccording to claim 11, wherein a channel forming region of the thirdtransistor comprises an oxide semiconductor.
 23. An electronic devicecomprising: a display portion having a pixel including a firsttransistor; a rechargeable battery being capable of charge bycontactless charge; a first antenna electrically connected to therechargeable battery; a memory device being configured to store data tobe applied to the display portion; and a second antenna electricallyconnected to the memory device, wherein the first antenna is capable ofreceiving electric power, wherein the second antenna is capable ofreceiving the data, wherein the rechargeable battery is configured tosupply electric power to the display portion and the memory device, andwherein the memory device includes: a second transistor, one of a sourceand a drain of the second transistor being electrically connected to abit line; a third transistor, one of a source and a drain of the thirdtransistor being electrically connected to a gate of the secondtransistor; and a capacitor, a first electrode of the capacitor beingelectrically connected to the gate of the second transistor and a secondelectrode of the capacitor being electrically connected to a word line.24. The electronic device according to claim 23, wherein a channelformation region of the third transistor comprises an oxidesemiconductor.
 25. The electronic device according to claim 23, whereinthe other of the source and the drain of the second transistor iselectrically connected to a first wiring, wherein the other of thesource and the drain of the third transistor is electrically connectedto a second wiring, and wherein a gate of the third transistor iselectrically connected to a third wiring.
 26. The electronic deviceaccording to claim 23, wherein the display portion comprises a liquidcrystal display device.
 27. The electronic device according to claim 23,wherein a channel formation region of the first transistor comprises anoxide semiconductor.
 28. The electronic device according to claim 23,wherein a channel formation region of the second transistor comprisessilicon.
 29. The electronic device according to claim 23, furthercomprising a driver circuit and a display controller electricallyconnected to the display portion, wherein operation of the drivercircuit and the display controller is stopped during a holding periodafter writing an image signal to the pixel.
 30. An electronic devicecomprising: a display portion having a pixel including a firsttransistor; a power source portion comprising a rechargeable batterybeing capable of charge by contactless charge; and a signal processingportion comprising a memory device, the memory device being configuredto store data to be applied to the display portion, wherein therechargeable, battery is configured to supply electric power to thedisplay portion and the signal processing portion, and wherein thememory device includes: a second transistor, one of a source and a drainof the second transistor being electrically connected to a bit line; athird transistor, one of a source and a drain of the third transistorbeing electrically connected to a gate of the second transistor; and acapacitor, a first electrode of the capacitor being electricallyconnected to the gate of the second transistor and a second electrode ofthe capacitor being electrically connected to a word line.
 31. Theelectronic device according to claim 30, wherein the display portioncomprises a liquid crystal display device.
 32. The electronic deviceaccording to claim 30, wherein a channel formation region of the firsttransistor comprises a first oxide semiconductor.
 33. The electronicdevice according to claim 30, wherein a channel formation region of thesecond transistor comprises silicon.
 34. The electronic device accordingto claim 30, further comprising a driver circuit and a displaycontroller electrically connected to the display portion, whereinoperation of the driver circuit and the display controller is stoppedduring a holding period after writing an image signal to the pixel. 35.The electronic device according to claim 30, wherein the other of thesource and the drain of the second transistor is electrically connectedto a second first wiring, wherein the other of the source and the drainof the third transistor is electrically connected to a second wiring,and wherein a gate of the third transistor is electrically connected toa third wiring.
 36. The electronic device according to claim 30, whereina channel formation region of the third transistor comprises a secondoxide semiconductor.